Its sometimes a indication of a swimbladder problem, does the koi sit at the bottom often? Eugen had / have a simular situation with a chagoi that came up for air but then sat at the bottom of the pond waiting to come up for its next breath.

Another possibility is that there is something on its gills that might be irretating it. For example i read that koi with parasite investations on the gills often blow bubbles over its gills to try and remove some of the parasites but it is actually pretty normal for this time of year... Heres a snippit from an article on my website that explains why.

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The [whitish] film/patches you see on your koi in winter are one of two things: Costia or, more likely, excess dead mucous coat. This is very common in winter and can be resolved with warm water and aeration. Ironically, it is the salt that assists in causing this build up as it first stimulates the mucous cells to work over time and then dries out the thickened mucous.

Normally koi swim, no correct that, ‘slide’ through the water. The mucous is worked backwards by this friction and leaves the body into the water column. This is a form of grooming for the koi but also serves the important function of casting off would-be foreign invaders. Finally, metabolites of the dermis and epidermis are lost through this action.

When a koi is in stasis, the normal slime removal is stopped. It is not unusual for it to become caked and ‘dead’ on the resting fish. Interestingly, these white patches always seems to be bilateral on the head (in crevices and creases of the skull and gill plate), a patch on crown or nose and along the body in islands.

When you warm fish up, you THEN can add some salt to stimulate normal slime production and encourage the sloughing of the old patches.

You might also notice that koi will blow bubbles over and out of the gills during this time. That is slime production caked on the lamella that they are grooming off.

Again, to an inexperienced eye, it is best to scrape this patch and make sure it is slime and epithelial cells and NOT costia or chilo.